1. Field of the Invention
The present invention relates to a method and apparatus for performing buffer status reporting (BSR), and more particularly, to a method and apparatus for accurately deciding a logic channel group corresponding to a short-format BSR control element when performing BSR.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, MAC provides transmission services via a plurality of logic channels. To manage uplink resources, when a user equipment (UE) performs uplink (UL) transmission, the network can divide or map logic channels of the UE into at most four groups based on priorities, types, and so on. The priorities are corresponding to values 1 to 8, and assigned by a higher layer, the radio resource control layer. Besides, the 3rd Generation Partnership Project, 3GPP, introduces a Buffer Status Reporting (BSR) procedure, which is used to provide the serving NB (or enhanced NB) with information about the amount of data in UL buffers of the UE.
In the BSR procedure, UE uses a MAC PDU (Protocol Data Unit) to carry a BSR control element, so as to report information about the amount of data in the UL buffers to the network. Accordingly, the network can determine the total amount of data available across one or all logical channel groups. According to different requirements, BSR control elements can be categorized into two formats: short and long. A short-format BSR control element is 1-byte long, and has 8 bits, where the former 2 bits indicate one logic channel group which buffer status is being reported, and the remaining six bits indicate the amount of data available across the logic channel group. A long-format BSR control element is three-byte long, and utilized for reporting data amount of the UL buffers of all the logic channel groups. Detailed description of the two formats can be found in related protocol specifications, and not narrated herein.
Via the long-format BSR control element, the network can recognized data amount of the UL buffers of all logic channel groups. However, the long-format BSR control element occupies much more bytes than the short-format BSR control element does, such that the long-format BSR control element is not suitable for the situation that the length of the MAC PDU is insufficient. For example, in some situations, UE can use a padding field of a MAC PDU to carry a BSR control element. If the padding field is not long enough, the short-format BSR control element is used. Since the short-format BSR control element carries information about the amount of data available across only one logical channel group, the MAC layer needs to select a logic channel group to be reported. However, the prior art does not disclose which logic channel group is corresponding to information carried by the short-format BSR control element.